There are different types of prior art welding power supplies. Two types of welding power supplies are phase controlled and inverter-based power supplies. Both types typically receive an ac line (60 Hz) input. However, inverter power supplies can be controlled to a desired frequency, but phase controlled power supplies are limited to the input frequency. Also, phase controlled power supplies cannot be used for pulse spray processes. Inverter-based power supplies are often preferred because they are lighter, have a faster response, provide better weld characteristics, and are better suited for multiple processes (MIG, TIG, stick etc.).
An inverter power supply receives a dc input (often called the dc bus), and switches the input to provide an ac output. Prior art inverter welding power supplies have been designed to receive a line frequency input (60 or 50 Hz), and to rectify that input to produce the dc bus.
The inverted ac output can be used as the welding output. However, some prior art welding power supplies include a rectifier which rectifies the ac inverter output to provide a dc welding output. The dc input to the inverter is typically obtained by rectifying an ac line input. Many inverter power supplies have controls which allow the power supply to effectively convert the ac line power into useful dc (and sometimes ac) welding power.
Engine driven generators used in welding are also common. An engine driven welding power supply is necessary for application where the user needs to weld at multiple locations and finds it necessary to move the welding power supply. An auxiliary power output (110 or 220 VAC) is usually provided for power tools, lights etc. Typically, engine driven generators are used to drive a simple tapped reactor or phase controlled power supplies. They often require an engine and generator specifically designed for the welding power supply, which can be more expensive than using a standard engine/generator. Phase controlled engine drive welding power supplies necessarily include all of the disadvantages of phase controlled power supplies.
Another prior art engine driven welding power supply is a dc welding power supply, wherein the dc output of the generator is used directly for a dc welding output. Such a welding power supply, with field control, is shown in U.S. Pat. No. 4,465,920, issued to Hoyt et al.
A few prior art inverter welding power supplies have been connected to a generator output and used as engine driven inverter welding power supplies. The generator ac output serves as the ac inverter input (which is rectified to create the dc bus). This arrangement creates many problems. First, inverter based welding power supplies have heretofore been designed to receive the relatively stable and constant ac line voltages. A generator does not always produce such a stable and constant output. Second, there has not been an integrated control system wherein the engine and or generator is controlled in response to the welding output or inverter operating parameters. Thus, these engines usually operate at full throttle constantly, and are very inefficient.
The common practice of providing an auxiliary power output on the generator has at least one disadvantage. The auxiliary power is single phase, 120 or 240 VAC at 50 or 60 Hz, and is used for power tools, lights etc. However, the single phase output unbalances the three phase output, and the result is harmonic distortion in all three phases. The distortion will cause one of the phases to have much higher peak voltage than the other two phases. The unusually high peak voltage may damage the inverter input capacitors, or require larger capacitors.
The distortion is caused by a backward component of a magnetic field wave. When a three phase load is present the three stator currents produce a magnetic field wave that rotates in the same direction as, and at the same speed as, the rotor. Thus, there is no relative motion between the rotor and the magnetic field wave, and the magnetic field wave does not induce any voltage in the rotor. However, when the load is unbalanced the magnetic field wave created by the stator currents does not move at the speed as and in the same direction as the rotor. The magnetic field produced by the stator currents when an unbalanced load is present may be resolved into two components: a forward component that is in the same direction and at the same speed as the rotor, and a backward component. The forward component behaves as a balanced three phase load, and does not cause a problem. The backward component is moving at the same speed as the rotor, but in the opposite direction. Thus, it has a motion relative to the rotor of twice the generator speed. This "moving" magnetic field will induce voltage in the rotor field winding, which causes the high output voltage. Damper cages have been used in generators (although no necessarily in the welding art) to counter-act or compensate for the effect of unbalanced loads.
Accordingly, it would be beneficial to have an inverter-based welding power supply that is engine driven where the control is integrated to control the engine and generator in response to either welding or inverter operating parameters. Preferably the generator will counter-act or compensate for the effect of unbalanced loads. Also, the power supply will preferably be able to be used for pulse spray and other welding processes.